The present invention relates to metal halide electric discharge lamps, that is to say, high-pressure electric discharge lamps having fills including mercury and metal halides.
Metal halide lamps have been known and manufactured for several years, and may exhibit an efficacy of about 65 Lm/watt through life, and a CIE general colour rendering index of about 72. For example, U.S. Pat. No. 3,407,327 to Koury et al discloses a metal halide lamp containing the halides of Hg, Sc, Th and an alkali metal, preferably Na.
Although the quality of the emitted light is satisfactory for many applications, the relative lack of red to deep red radiation as compared with other artificial light sources can make a conventional metal halide lamp unsuitable for certain critical applications. For example, satisfactory rendering of the colours of textiles and of fresh reddish foodstuffs, especially meat, demand a higher proportion of red radiation than is given by a standard sodium iodide-scandium iodide-caesium iodide-thorium iodide lamp, even when the lamp is provided with the most efficient red emitting phosphor known.
U.S. Pat. No. 3,911,308 to Akutsu et al teaches the addition of alkali metal halide, but especially sodium iodide, to scandium iodide lamps for the purpose of arc stabilization, lowering of the re-ignition voltage and improving the colour rendering. The use of alkali metals such as Na or Cs in stabilizing arcs in metal halide lamps is also described by Koury et al in U.S. Pat. No. 3,262,012.
U.S. Pat. No. 3,234,421 to Reiling discloses generally metal iodide lamps including iodides selected from those of Li, Na, Cs, Ca, Cd, Ba, Hg, Ga, In, Tl, Ge, Sn, Th, Se, Te and Zn, but is especially concerned with lamps containing sodium and thallium iodides. Lithium iodide is, however, corrosive and is known to attack the wall of the discharge vessel when employed in, for example, the sodium-indium-thallium iodide lamp described in British Pat. No. 1,125,063 or, indeed, in a high pressure lamp having a fill of mercury alone and no metal halide, as discussed in British Pat. No. 1,400,976.
U.S. Pat. No. 3,979,624 to Liu et al disclose metal halide lamps containing sodium and/or lithium iodides as well as scandium iodide, in which the molar ratio of alkali metal halide to scandium halide is from 1.7:1 to 5:1, resulting in enhanced efficacy, and teach that enhanced efficacy requires a low alkali metal to scandium ratio (below 5:1), contrasting this with the lamps of Koury et al No. 3 407 327, which show a NaI:ScI.sub.3 molar ratio in excess of 11.5. The lamps of Liu et al do, however, have an undesirably high correlated colour temperature (CCT) for interior lighting applications, quoting a CCT of 5557.degree. K. for their examples with Na and Li (column 6, lines 26-59) with NaI+LiI:ScI.sub.3 ratios of 2:1 and 4.2:1. Their examples contain, respectively, 34.04 mole % LiI, 32.58% NaI and 33.33% ScI.sub.3, 45.11% LiI, 21.56% NaI and 33.33% ScI.sub.3, and 26.07% LiI, 54.66% NaI and 19.26% ScI.sub.3.